1. Field of the Invention
The present invention relates to an electrical coil, particularly a gradient coil for a magnetic resonance apparatus.
2. Description of the Prior Art
The technical range of application of electrical coils is versatile. In many designs, an electrical coil has a casting, for example from a artificial resin, in order to, among other things, obtain a high insulating strength and a high structural strength. Moreover, it is known to cool an electrical coil during operation of the coil in order to, among other things, increase the efficiency. For this purpose, a cooling device, for example, transports heat arising in a conductor of the coil as a result of a current flow out of the coil.
A highly stressed electrical coil is a gradient coil of a magnetic resonance apparatus, for example. Among other things, the magnetic resonance apparatus has a gradient coil system for generating rapidly switched gradient fields, as well as a basic field magnetic system for generating a static basic magnetic field. The gradient coil system often contains means for reducing non-homogeneity of the static basic magnetic field, referred to as shim devices. Given a passive shim device, a number of iron sheets are introduced into a suitable arrangement in the gradient coil system, for example. For this purpose, the basic magnetic field is measured before the iron sheets are inserted, and a calculating program determines the appropriate number and arrangement of the iron sheets.
The amplitudes of the required currents in the conductor of the coil are several hundred amperes during the operation of the gradient coil. The current increase rates and current decrease rates are several 100 kA/s. The driving voltage for the coil current is up to several kV. The gradient coil is frequently cooled for controlling the aforementioned high electrical performances. For example, German OS 197 21 985 and
German OS 197 22 211 disclose a cooling device for indirectly cooling conductors of the gradient coil. A flexible cooling line, is interleaved with the conductors of the gradient coil system, in a cylindrical jacket which is the filled with resin. A cooling medium is fed through the cooling line for cooling the gradient coil.
For example, German OS 198 39 987 describes another embodiment for cooling a gradient coil. A conductor of the gradient coil is directly cooled by feeding a cooling medium through an inner cooling channel, which is surrounded by the conductor as a profile.
It is known from U.S. Pat. No. 5,786,695, for example, that a constant temperature of a passive shim device is important for a constant accuracy of a shim effect. The heat build-up in the conductor of the gradient coil leads to a change in temperature of the passive shim device, so that the homogeneity of the basic magnetic field and therefore the quality of magnetic resonance images is impaired. In order to prevent the aforementioned temperature fluctuations, the aforementioned patent teaches arranging the shim device in the gradient coil system such that it can be cooled by a circuit coolant for obtaining a high temperature stability.
An object of the present invention is to provide an improved coolable electrical coil.
This object is achieved in accordance with the invention in an electrical coil, particularly a gradient coil for a magnetic resonance apparatus, having at least one electrical conductor, a carrier structure, at least one component of a cooling device and a heat insulator, which is arranged between the conductor and the carrier structure for at least one section of the conductor.
As a result, the conductor of the coil can be operated at high temperatures without the carrier structure, such as a resin casting, simultaneously assuming dangerously high temperatures, which advantageously leads to a reduced thermal expansion of the carrier structure and increases the time stability of the gradient fields and of the basic magnetic field, particularly with respect to a magnetic resonance device. Since the conductor can assume high temperatures relative to the carrier structure, a cooling medium flow of the cooling device can be operated with a high temperature drop, so that high power densities are possible within the coil.
In an embodiment, at least one section of the conductor is hollow-cylindrically fashioned for conducting a cooling medium. For example, the aforementioned German OS 198 39 987 describes an embodiment of the aforementioned conductor interior cooling. In particular, an embodiment of the conductor as a hollow cylindrical conductor also achieves efficient high-frequency properties, for example regarding the skin effect.
In another embodiment, the component of the cooling device is fashioned for cooling at least one section of the conductor. As a result of the efficient thermal conductivity of the conductor, for example when the conductor is composed of cooper or aluminum, the component of the cooling device is only sufficient for one section of the conductor in order to obtain an efficient cooling effect for a larger section of the conductor. Sections of the conductor, which are not provided with the component of the cooling device, exhibit heat insulation given the aforementioned cooling by sectors. As a result, temperatures and temperature fluctuations, which occur in heat-insulated conductor sections being comparably far away from the location of cooling and which are greater compared to the cooling location, do not have a disadvantageous effect on the surrounding carrier structure. On the basis of the aforementioned cooling, which is only fashioned in sections, a correspondingly simple cooling device can be fashioned.
For this purpose, the section to be cooled of the conductor, in an embodiment, extends in an edge region of a spatial expanse of the coil, for example, given a spatial expanse corresponding to a hollow cylinder, in a region of a front side of the hollow cylinder. As a result of simple accessibility and short runs, the cooling device can be particularly simply and economically fashioned. Furthermore, space is available for other components in a central area.
In a further embodiment, the heat insulator encloses the conductor. A heat insulation of the conductor from all sides is thus achieved.
In another embodiment, the heat insulator exhibits less thermal conductivity than the carrier structure, the thermal conductivity of the heat insulator, for example, is greater by one to three factors less than the thermal conductivity of the carrier structure. A carrier structure that is primarily fashioned from a resin casting has a thermal conductivity of greater than approximately 0.15 W/(Kxc2x7m), for example. The heat insulator can be fibrous material and/or high-resistance foam material containing one or more of glass, ceramic, mineral materials and/or polymer materials, such a heat insulator exhibits heat conductivities of approximately 0.05 W/(Kxc2x7m) and less.
In a further embodiment, the carrier structure includes an arrangement for reducing non-homogeneity of a magnetic field, such as the above-described passive shim devices. The same is true for this arrangement as described above for the carrier structure. A complicated separate cooling of this arrangement for obtaining a high temperature stability is not necessary.